Under ordinary circumstances, the thermal regulatory system of the human body maintains a near constant temperature of about 37□ C. (98.6□ F.). Heat lost to the environment is precisely balanced by internal heat produced within the body.
Hypothermia is a condition of abnormally low body temperature generally characterized by a core body temperature of 35□ C. or less, and may be further clinically defined according to its severity. For example, a body core temperature within the range of 32□ C. to 35□ C. may be described as mild hypothermia, 30□ C. to 32□ C. as moderate, 24□ C. to 30□ C. as severe, and a body temperature of less than 24□ C. may constitute profound hypothermia. Although the above ranges may provide a useful basis for discussion, they are not absolutes and definitions vary widely as indicated in the medical literature.
Hyperthermia may be defined as a condition of abnormally high body temperature, and may be the result from exposure to a hot environment or surroundings, overexertion, or fever. Body core temperatures may range from 38□ C. to 41□ C. due to conditions such as fever, and may be substantially higher in cases of exposure and overexertion. Like hypothermia, hyperthermia is a serious condition that can be fatal.
Although both hypothermia and hyperthermia may be harmful and require treatment in some case, in other cases hyperthermia or hypothermia, and particularly hypothermia, may be therapeutic or otherwise advantageous, and therefore may be intentionally induced. For example, periods of cardiac arrest in the setting of myocardial infarction and heart surgery can produce brain damage or other nerve damage. Hypothermia is recognized in the medical community as an accepted neuroprotectant during cardiovascular surgery and therefore a patient is often kept in a state of induced hypothermia during cardiovascular surgery. Likewise, hypothermia is sometimes induced as a neuroprotectant during neurosurgery. Hypothermia may also be beneficial in other situations, for example, for victims of head trauma, spinal trauma, brain attack (also sometimes called stroke), spinal surgery or surgery where blood flow may be interrupted or compromised to the brain or spinal cord such as aneurysm repair, as well as other types of surgery where neuroprotection is desirable.
Neural tissue, that is all tissue of the nervous system such as the brain or spinal cord, is particularly subject to damage by vascular disease processes including, but not limited to ischemic or hemorrhagic stroke, blood deprivation for any reason, including cardiac arrest, intracerebral hemorrhage and head trauma. In each of these instances, damage to brain tissue may occur because of ischemia, pressure, edema or other processes resulting in a loss of cerebral function and permanent neurological deficits. Lowering the brain temperature may confer neuroprotection through several mechanisms including the blunting of post-insult elevation of neurotransmitters such as glutamate, reduction of cerebral metabolic rate, moderation of intracellular calcium, prevention of intracellular protein synthesis inhibition, and reduction of free radical formation as well as other enzymatic cascades and even genetic responses. Thus intentionally induced hypothermia may prevent some of the damage to brain or other neurological tissue during surgery or as a result of stroke, intracerebral hemorrhage and trauma.
Treatment of stroke in particular is a possibly therapeutic use of intentionally induced hypothermia. Stroke (sometimes called brain attack) is a severely debilitating and complex disease that results from the blockage (ischemic stroke) or rupture (hemorrhagic stroke) of a blood vessel within or leading to the brain region. During a stroke, brain cells are damaged either by a lack of oxygen or by increased pressure. These events can eventually result in death and necrosis of brain tissue. In general, at least one goal in the therapeutic intervention for stroke is to preserve the function of as much brain tissue as possible. However, current medical treatment for stroke is largely supportive in nature. Newer treatments, for example clot-dissolving drugs, are available but may be only suitable for treatment of ischemic strokes and must generally be used shortly (within several hours) of the initial stroke symptoms to avoid side effects related to bleeding within the brain. In practice, it has been difficult to treat strokes within this time window since patients often do not arrive at a medical facility until several hours after the onset of a stroke. As a result, most strokes are not aggressively treated with medical therapy. A treatment to prolong this time window, and to protect brain cells from death, would have a profound impact on patient care.
Experimental studies of ischemia have shown reduction in infarcted brain tissue volume in animals treated with hypothermia during or shortly after a stroke or ischemic insult. It is therefore believed that the application of hypothermia to a patient who is suffering or has recently suffered a stroke may be beneficial.
Despite the acceptance of hypothermia as a neuroprotectant, it has not been widely used outside of the surgical setting. Additionally, most current practices attempt to provide hypothermia to the brain by inducing whole body hypothermia through systemic treatment. However, whole body hypothermia presents numerous difficulties and is cumbersome to implement in a patient who is not under general anesthesia. Lowering the systemic temperature of a patient not only takes a significant amount of time, but also subjects the patient to deleterious effects of hypothermia including cardiac arrhythmias, coagulation problems, increased susceptibility to infections, and problems of discomfort such as profound shivering.
Control of the body's temperature, for example, to maintain normothermia (usually 37□ C.), is often desirable. For example, in a patient under general anesthesia, the body's normal temperature regulating mechanisms may not be fully functioning, and the anesthesiologist may be required to artificially control the patient's body temperature. Similarly, a patient may lose an extraordinary amount of heat to the environment, for example, during major surgery, and the patient's unaided body may not be able to generate sufficient heat to compensate for the heat lost. A device and method for controlling body temperature, for example by adding heat to maintain normothermia, would be desirable.
Particularly in the surgical setting, it has sometimes been the case that blood or other fluid was heated or cooled outside a patient's body and introduced into the body to heat or cool the body or some target location within the body. However, heating or cooling fluids outside of the patient may be cumbersome and require elaborate equipment. For example, in surgery, the temperature of a patient may be controlled by a bypass machine where a significant amount of the patient's blood is removed, heated or cooled outside the body in a by-pass machine, and reintroduced to the patient's blood stream. One particular application of this procedure is whole body hypothermia sometimes induced during heart surgery. Other examples include hypothermia induced during neurosurgery or aortic or other vascular surgery.
The use of an external method for inducing hypothermia, such as a bypass machine, is an extremely invasive procedure that subjects vast quantities of the patients' blood to pumping for an extended length of time. External pumping of blood may be harmful to the blood, and continued pumping of blood into a patient for extensive periods of time, for example, more than one or two hours, is generally avoided. Additionally, such a procedure may require systemic treatment of the patient, for example, with heparin to prevent clotting which may present other undesirable consequences in a stroke victim.
Means of imparting heat to the blood of a patient, or removing heat from the patient, which do not require external pumping have been proposed. For example, one particular catheter structure which has been developed to treat patients suffering from either hypothermia or hyperthermia is described in U.S. Pat. No. 5,486,208, to Ginsburg, the complete disclosure of which is herein incorporated by reference. That patent issued from one of the applications from which this application claims priority. A catheter disclosed in that patent was inserted into a blood vessel and a portion of the catheter heated or cooled, transferring heat to the patient's blood and thereby affecting the overall body temperature of the patient. However, while such devices and methods may avoid the problems associated with external pumping of blood, they do not eliminate the difficulties that arise when the entire body is subjected to hypothermia.
There have been attempts to achieve regional cerebral hypothermia, for example by placing the head in a cooled helmet or shroud, or even injecting a cold solution into the head region. Attempts to achieve brain cooling by directly cooling the surface of the head have proven impractical or ineffective because of factors such as the insulating qualities of the skull, which make it difficult to effectively lower brain core temperature, and the blood flow that may fail to provide sufficient heat transfer circulation to the brain itself when the surface of the head is cooled. Patients, especially patients not under general anesthesia, may also find it difficult to tolerate immersion or direct exposure of the head to a cold solution or cooling surface.
An apparatus to facilitate transfer of heat to or from a target location by means of internally applied heating or cooling would be advantageous. It has been known in the art to impart heat by direct contact with specific tissue by means of a balloon catheter. For example, in U.S. Pat. No. 5,019,075 to Spears, a heated balloon was described to apply heat directly from the surface of the balloon to the wall of an artery dilated during percutaneous transluminal coronary angioplasty (PTCA) to fuse together disrupted tissue. This device, however, operated by direct contact between the vessel wall in question and a greatly heated balloon surface.
Balloons capable of acting as ongoing heat transfer balloons by the continual flow of heat transfer medium through the balloon have also been shown. For example, in U.S. Pat. No. 5,624,392 to Saab, a concentric inflow and outflow lumen each terminate within the heat transfer balloon so that a continual flow of heat transfer liquid can be maintained within the balloon for controlled heat transfer to the adjacent tissue.
U.S. Pat. No. 5,269,758 to Taheri, discloses a balloon in which heated fluid such as heated saline solution is circulated through a balloon that pulses. The heat from the heat transfer liquid may then be imparted to the blood as it flows past the balloon to treat hypothermia in a patient. The flow of the affected blood is not otherwise generally directed nor is the temperature of a target region disclosed to be altered by the heated balloon of Tahari.
The configuration of balloons to provide channels for the flow of blood from the proximal side to the distal side of a balloon blocking a blood vessel, such as a balloon used for PTCA has also been shown. For example, such an autoperfusion balloon angioplasty catheter is shown in U.S. Pat. No. 4,581,017 to Sahota, and the multi-lumen balloon shown in U.S. Pat. No. 5,342,301 to Saab as discussed for use in angioplasty discloses a multi-lumen balloon catheter configured to allow blood to perfuse from the proximal side to the distal side of a balloon angioplasty catheter when the balloon is inflated to apply angioplastic pressure against the blood vessel walls and otherwise fully obstruct blood passage.
It would be desirable to devise an apparatus capable of heating or cooling liquid such as blood within the body and directing that liquid after it is heated or cooled, to a target location. It would be particularly advantageous if a device could be devised where the liquid could be directed to a desired location using only the patient's own heart as a pump. It would also be particularly advantageous if a method could be devised for directing heated or cooled blood to a target region of a patient's body for a sufficient length of time to affect the temperature of that target region.
A method of treating a patient to protect tissue, and particularly neural tissue, by inducing hypothermia is desirable. Protecting particular target tissue by inducing hypothermia in that tissue by means of in situ cooling of body fluid directed to that tissue would be particularly advantageous.
It would also be desirable to provide a system to control such a device to perform the method of treatment in a simple and predictable manner. It would be particularly desirable if such a system could control the device in conjunction with feedback data from a patient to control the device to predictably and selectively affect the temperature of a target region in the patient.